1. Field of the Invention
The invention relates to an optical read/write device for an information medium. By way of example, this medium may be an optical medium or a magnetic or magnetooptic medium, such as a magnetic disk.
2. Discussion of the Background
FIG. 1b shows a conventional system for reading/writing on a magnetic medium. In such a system, a laser source S1 emits a light beam which is focused by lenses L1 and L2 onto the recording medium D1. This is a magnetooptic ferromagnetic material such as, for example, an alloy of rare earths (for example a terbium/iron alloy). This material has the property of acting on the polarization of the light which it receives depending on its magnetization. The light beam is reflected by the recording medium by possibly having its polarization modified by interaction with the magnetization of the information medium. The beam reflected by D1 is partly routed by the beam splitter SP1 to the beam splitter SP2. The splitter SP2 is a polarization splitter which routes one polarization to the photodetector PD and the other polarization to the other photodetector PP.
For recording, such a system records by localized heating using a beam, of sufficient energy level, transmitted by the source 1. Owing to the effect of a magnetic field B applied to the recording medium, the latter, as it cools, has its magnetization locally oriented in the direction of the applied magnetic field.
In such a system, the minimum size of a unit of information is diffraction limited. It is equal to approximately .lambda./2 (NA)
.lambda. being the wavelength of the light beam and PA1 NA being the numerical aperture of the beam. PA1 a) as may be seen in FIG. 1, the great reduction in diameter of the waveguide near its end defines an evanescent region, that is to say a region of the waveguide in which the optical wave is no longer propagated but is evanescent. The amplitude of an optical wave propagating towards the stop, or emanating therefrom, will therefore experience, on passing through this region, an attenuation by a factor of e.sup.-d/.delta., where d is the length of the region and .delta. is a characteristic attenuation length depending on the detailed structure of the waveguide; PA1 b) the transmission or collection efficiency of the stop, defined as the ratio of the collected or transmitted power to the product formed by multiplying the incident power density by the area of the stop, is proportional to (a/.lambda.).sup.4 (Rayleigh's law for diffraction by objects which are small compared with .lambda.), where a is the radius of the stop (by definition, small compared with .lambda.) and .lambda. is the wavelength used. PA1 a transparent substrate having an approximately plane face carrying at least one pair of electrodes (E1-E2) defining an airgap area whose size corresponds approximately to the information item to be written or to be read, a pair of electrodes (E1-E2 or E3-E4) constituting a resonator for an incident electromagnetic wave having one component of its electric field parallel to the direction of alignment of the electrodes of the said pair of electrodes; PA1 an optical source illuminating the said electrodes with an optical beam, one component of the electric field of which is parallel to the direction of alignment of the pair of electrodes.
The size of the information item, and therefore the density of the information recorded, is limited by the wavelength of the beam. To reduce the size of the beam's spot on the information medium, it is possible in particular to make use of techniques of the type used in near-field optical microscopy.
The most conventional technique in near-field optical microscopy consists in the use of a stop of size smaller than the diffraction spot. If the stop is placed near enough to the object to be observed, the resolution obtained is essentially equal to the size of the stop. Near-field optical microscopes based on this principle have been described and produced by various teams (see the articles by D. W. Pohl et al., Applied Physics Letters 44, 652 (1984) and E. Betzig et al., Applied Physics Letters 51, 2088 (1987)).
Although this approach has allowed resolutions of the order of 50 nm to be demonstrated with illumination wavelengths of the order of 500 nm (resolution.apprxeq..lambda./10), it suffers from a fundamental difficulty, namely the very low level of optical signal that can be detected.
In practice, and so as to be able in particular to image surfaces which are not perfectly plane, the stop is physically produced at the end of an optical fibre or of a micropipette which is metallized and drawn so as to be thinned down at its end, as may be seen in FIG. 1a.
Such a probe for near-field microscopy, which may be regarded as a metallized electromagnetic waveguide of variable diameter, is generally brought up to and moved in the vicinity of the surface to be imaged using piezoelectric actuators, it being possible for various servocontrol signals to be generated so as to perform a scan at a constant height.
Three modes of operation are possible:
1) Operation in transmission mode
The waveguide is connected at the opposite end from the stop to a light source, generally a laser. That part of the light which is coupled into the waveguide propagates as far as the stop, where a small fraction is transmitted. If the surface of an at least partly transparent object is observed, a conventional optical system may collect the light transmitted through the object and direct it onto a photodetector. The signal thus photodetected while the surface is being scanned by the probe allows a point-by-point image to be reconstructed.
2) Operation in collection mode
This imaging mode consists in illuminating the imaged surface using a conventional optic operating by transmission through the object. The end of the probe is used for collecting the optical near field in the vicinity of the surface. The light thus coupled into the probe is then taken to a photodetector.
3) Operation in reflection mode
In this third and last mode, which may be seen as a combination of the two previous ones, the probe serves at the same time both to illuminate and to collect. An optical system, for example one having a semi-transparent component, allows that end of the probe on the opposite side from the stop to be connected simultaneously to a light source and to a photodetector. The variations in the power reflected by the end of the fibre are then exploited.
None of these three modes allows the acquisition of high-quality signals, for two reasons:
Under the conditions that can be used in practice, these two conjugated factors result in the detection of optical powers, which have effectively interacted in the near field with the surface, of the order of 10 nW for illuminations of the order of a few tens of mW (the usable limit above which destruction of the probe is observed). For a degree of modulation of 5%, associated with local variations in properties of the surface observed, and at a wavelength of 500 nm, this results, assuming that the detector noise is negligible, in a signal-to-noise ratio of 78 dB in a 1 Hz bandwidth, which is reduced to 18 dB for a 1 MHz bandwidth. It may thus be seen that the acquisition of an image comprising 1,000.times.1,000 pixels cannot be envisaged in less than 1 second.
The use of such near-field microscopy probes, as an optical, and in particular magnetooptical, read device for a surface of an information medium therefore cannot be envisaged for high data-transfer rates (.gtoreq.10 Mb/s) in read mode.
The very low powers available at the end of these probes also constitute an obstacle to the use of a write method based on local heating, as described above.
Moreover, the very structure of the probes described--optical fibres or pipettes drawn and then metallized--means that they have to be manufactured on an individual basis, something which poses reproducibility problems (as has been seen, the efficiency of the probe varies as the 4th power of the size).